The present invention relates to a method of operating a controller for a power converter, and a corresponding controller.
Energy harvesting enables remote sensors of a wireless sensor network to obtain power from the environment for their entire operational lifetime. For indoor remote sensors, amorphous silicon photovoltaic (PV) cell can be used to harvest energy from indoor lighting, thus functioning as an energy-harvesting (EH) source. Furthermore, if the power consumption of the sensor is low, e.g., the image sensor in [1], the power rating of the PV cell is limited to tens or hundreds of microwatts to minimize the form factor of the sensor. However, as the output power of the PV cell varies greatly with illumination level [2] and the output voltage of the PV cell (VPV), an energy storage (ES) device, such as a battery, is required to regulate the harvester's output power. Furthermore, a DC-DC converter with a maximum power point tracker (MPPT) is needed to lock the PV cell at its maximum power point (MPP).
Image sensors usually require a high supply voltage (e.g. 1.8V in [1]), to achieve sufficient sensitivity. However, digital blocks need to ideally operate at 1V or less to minimize power consumption. As a result, the remote sensor would require at least two power rails to optimize its performance. In [2-4], harvested energy only recharges the battery, thus requiring an additional power converter to deliver energy from the battery to the load (and elaborated below). So, two steps of power conversion are required, which reduces the overall conversion efficiency.
Pulse-Skipping Modulation (PSM) Controller
In [2], an inductive power converter is implemented, which is able to regulate output voltages to a continuous range of values. The controller of the power converter operates with pulse-skipping modulation (PSM) to achieve a low static power consumption of 1.95 μW. Specifically, the controller of the PSM is configured to skip several clock pulses between two switching activities to minimize switching losses when the power level is low. The number of clock pulses to be skipped depends on an amount of power harvested from the PV cell and the required voltage level for maximum power point tracking of the PV cell. A nano-power reference circuit and a relaxation oscillator are also included in the controller.
Proportional-Integral (PI) Controller
Since the power harvested can reach as low as several microwatts, a challenge is to design the controller of a power converter to consume minimal power to ensure high efficiency. Furthermore, the power converter needs to be capable of handling a large range of input and output power as the harvested power may vary by several orders of magnitude, depending on environmental conditions. For example, in [3], the power converter is implemented with two Pulse-Width Modulation (PWM) converters using proportional-integral control. As the design uses several integrated circuits and the conventional proportional-integral control, the power consumption of the said controller is relatively high, at 135 μW.
Switch-Capacitor Power Converter
In [4], a switch-capacitor power converter architecture is implemented with a controller power consumption of 2.4 μW. The said power converter is controlled by a relaxation oscillator, which varies its switching frequency accordingly to the power level. As such, the switching frequency can be reduced when the harvested power is low to reduce power consumption in the controller.
For this design, a linear voltage regulator is used to regulate the power supply of the oscillator, which consumes a significant amount of DC power regardless of the power level. Moreover, the relaxation oscillator, as configured, requires two reference voltages to be generated and two comparators to perform voltage comparisons. As both the reference voltages and comparators consume a considerable amount of static power, further reduction in controller power consumption is limited. Moreover, the switch-capacitor power converter architecture can only regulate the output voltage to discrete levels.
Dual-Input-Dual-Output Power Converter
In the three above designs described, the respective power converters are configured to only transfer the input power from the EH source (e.g. a PV cell) to recharge the ES device (e.g. a battery). As a result, an additional power converter is needed to interface between the energy storage and the load (LD) and thus inadvertently reduces the overall power efficiency of the associated system. In [5], the power converter is capable of transferring the harvested power to both the ES source and the LD in one conversion step, thereby improving the overall power efficiency. However, the controller in [5] uses lead-lag compensation, which requires error amplifiers, and comparators. Both the error amplifiers and comparators consume a significant amount of static power and undesirably result in relatively high controller power consumption.
One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art.